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“”I would say that before creation, there is God. But if God has never existed and does not exist now, then do you say He is real? Or, a recent example that has been used is unicorns on Mars. We would all say that they do not exist. Is there a difference?””

Since I'm into and enjoying the study of Blessed Aquinas these days...

This should help(I hope)

That in Created Subsistent Intelligences there is a
Difference between Existence and Essence

THOUGH subsistent intelligences are not corporeal, nor compounded of matter and form, nor existent as material* forms in matter, still it must not be thought that they come up to the simplicity of the being of God: for there is found in them a certain composition, inasmuch as existence (esse) and essence (quod est) is not in them the same.*
4. Whatsoever reality subsists of and by itself, nothing attaches to that reality except what is proper to being as being. For what is said of any reality not as such, does not belong to that reality otherwise than accidentally by reason of the subject:* hence, considered apart from the subject in a particular case, the attribute does not belong to that reality at all. Now to be ‘caused by another’ does not belong to being, as being: otherwise every being would be caused by another, which is impossible (B. I, Chap. XIII) Therefore that existence which is being of itself and by itself, must be uncaused. No caused being therefore is its own existence.

5. The substance of every reality is a being of itself and not through another. Hence actual illumination is not of the substance of air, because it accrues to it through another. But to every created reality existence accrues through another, otherwise it would not be a creature. Therefore of no created substance is it true to say that its existence is its substance.*

Hence in Exodus iii, 14, existence is assigned as the proper name of God, He who is: because it is proper to God alone that His substance is none other than His existence.

An added bonus to wrap your mind around

That in God there is no Composition

IN every compound there must be actuality and potentiality. For a plurality of things cannot become one thing, unless there be actuality and potentiality. For things that are not one absolutely, are not actually united except by being in a manner tied up together or driven together: in which case the parts thus got together are in potentiality in respect of union; for they combine actually, after having been potentially combinable. But in God there is no potentiality: therefore there is not in Him any composition.*
3. Every compound is potentially soluble in respect of its being compound, although in some cases there may be some other fact that stands in the way of dissolution. But what is soluble is in potentiality not to be, which cannot be said of God, seeing that He is of Himself a necessary Being

That God in willing Himself wills also other things besides Himself*

EVERY one desires the perfection of that which for its own sake he wills and loves: for the things which we love for their own sakes we wish to be excellent, and ever better and better, and to be multiplied as much as possible. But God wills and loves His essence for its own sake. Now that essence is not augmentable and multipliable in itself (Chap. XLII), but can be multiplied only in its likeness, which is shared by many. God therefore wills the multitude of things, inasmuch as He wills and loves His own perfection.
3. Whoever loves anything in itself and for itself, wills consequently all things in which that thing is found: as he who loves sweetness in itself must love all sweet things. But God wills and loves His own being in itself and for itself; and all other being is a sort of participation by likeness of His being.

6. The will follows the understanding. But God with His understanding understands Himself in the first place, and in Himself understands all other things: therefore in like manner He wills Himself in the first place, and in willing Himself wills all other things.

This is confirmed by the authority of Holy Scripture: Thou lovest all things that are, and hatest nothing of the things that thou hast made (Wisd. xi, 2)